Conventionally, as a porous material having Si—O bonds, there has been known zeolite or silica gel. Zeolite is a silica crystal having uniform pores, but there is no zeolite that has the pore diameter of exceeding 13 Å. Furthermore, silica gel has pores having the meso area in the range of 2 to 50 nm, but its pore size distribution is not uniform. Accordingly, these materials have only been used for limited purposes.
Contrary to this, a porous inorganic compound having uniform mesopores has uniform pores in the meso area, and has a large pore volume and a large surface area so that it has been expected to be used for a catalyst carrier, a separation adsorbent, a fuel battery and a sensor.
As for a process for preparing such an oxide having uniform mesopores, a process utilizing control of the structure of an inorganic material by the use of an organic compound has been paid attention because an oxide of novel shape and structure can be obtained. In particular, an oxide having uniform mesopores, which is synthesized by utilizing self-assembly of an organic compound and an inorganic compound, is known to have a larger pore volume and a larger surface area than conventional oxides. An oxide having uniform mesopores mentioned herein refers to one in which existence of the diffraction peak showing a periodic arrangement of a structure is confirmed by measurement according to the X-ray diffractometry because pores are regularly disposed in an oxide (a periodic pore structure).
As a process for preparing an oxide having uniform mesopores utilizing self-assembly of an organic compound and an inorganic compound, there is described a process, for example, in W091/11390. In the cited reference, a process comprising subjecting silica gel, a surface active agent or the like to hydrothermal synthesis reaction in a closed heat-resistant vessel to prepare such an oxide is described. Furthermore, in Bull. Chem. Soc. Jp., Vol. 63, p. 988 (1990), a process comprising subjecting kanemite that is a kind of a layered silicate and a surface active agent to ion exchange to prepare such an oxide is described.
As an oxide having uniform mesopores prepared by this method has recently been used for an optically functional material, an electronically functional material or the like, it has been prepared in a shape of a film.
For example, in Nature, Vol. 379, p. 703 (1996) and J. Am. Chem. Soc., Vol. 121, p. 7618 (1999), a process comprising immersing a substrate in a sol-containing solution including a condensate of alkoxysilane and a surface active agent, and precipitating porous silica on a surface of a substrate to form a film is described. In addition, in Supramolecular Science, Vol. 5, p. 247 (1998), Adv. Mater., Vol. 10, p. 1380 (1998), Nature, Vol. 389, p. 364 (1997) and Nature, Vol. 398, p. 223 (1999), a process comprising coating a substrate with a solution in which a condensate of alkoxysilane and a surface active agent are dissolved in an organic solvent and subsequently evaporating the organic solvent to form a film on the substrate is described.
Among these methods, the former process comprising precipitating porous silica on the surface of the substrate has disadvantages such that it requires a long time to form a film and porous silica is precipitated in a shape of powder in many cases, thus resulting in reducing the yield. For this reason, to form a porous silica film, the latter process comprising evaporating the organic solvent is excellent.
In the process comprising evaporating the organic solvent to form a film on a substrate, examples of a solvent used, including a polyhydric alcohol ether solvent, a glycol ether acetate solvent, an amide series solvent, a ketone series solvent, a carboxylic acid ester solvent and the like are cited in JP2000-38509A. Furthermore, solvents such as an organic solvent having amide bonds and an organic solvent having ester bonds are cited in WO99/03926.
On the other hand, in late years, when this porous silica film is used for an optically functional material, an electronically functional material or the like, there has been a problem that both low moisture adsorption property and high mechanical strength of a film are needed. For example, a porous silica film, when used for an interlayer insulating film in a semiconductor as an electronically functional material, is favorable as a film with a very low relative permittivity because the ratio of a pore having the relative permittivity of 1 is high, but being porous, such a film considerably deteriorates the mechanical strength.
For this reason, as a process for preventing water from adsorbing, a process comprising introducing a hydrophobic functional group into an interlayer insulating film material has been proposed. However, a process for improving the mechanical strength at the same time has not been reported up to now. For example, in WO00/39028 and U.S. Pat. No. 6,208,014, a process for preventing water from adsorbing by trimethylsilylating silanol groups in the pore has been proposed. However, it is not possible to completely trimethylsilylating silanol groups in the pore with this method, which has been reported in J. Phys. Chem., Vol. B1997-101, p. 6525 and J. Colloid Interface Sci., Vol. 188, p. 409 (1997). Further, regarding the mechanical strength, the process does not have any effect, which has been reported in J. Electrochem. Soc., Vol. 150-6, p. F123 (2003).
Furthermore, in JP2001-049174A, a process for forming a porous silica film by a coating solution prepared by using a copolycondensate (cogelated product) of methyltrialkoxysilane and tetraalkoxysilane has been proposed. This process improves the hydrophobic property of the porous silica film obtained by using a coating solution in which the content of methyltrialkoxysilane, i.e., a hydrophobic component, is increased. However, when the content of methyltrialkoxysilane is increased, the ratio of three dimensional bond unit of Si—O—Si bonds forming a skeleton of the porous silica film is reduced; therefore, the mechanical strength is remarkably deteriorated. Accordingly, it has been difficult to have both the hydrophobic property and the mechanical strength at the same time.
In addition, in Chem. Commun., p. 1487 (2000), a process for producing a hydrophobic mesoporous silica powder by partially hydrolyzing dimethylalkoxysilane and tetraalkoxysilane respectively and then mixing them has been reported. A powder obtained by this process has a periodic pore structure even if dialkylalkoxysilane is introduced in a relatively high amount and has the excellent hydrophobic property. However, in this process, as it takes several days for the preparation, it is not practical. Further, as a powder is finally obtained, it is difficult to apply it to an optically functional material, an electronically functional material or the like.
Furthermore, in The Journal of Surface Science Society of Japan, Vol. 22, p. 9 (2001), it has been reported that a powder obtained by thin coating of tetramethyltetracyclosiloxane, i.e., a cyclic siloxane compound on a powder surface exhibits the hydrophobic property. In this case, however, as a powder is finally obtained, it is difficult to apply it to an optically functional material, an electronically functional material or the like.
Furthermore, in U.S. Pat. No. 5,939,141, a process for making hydrophobic by forming a film by vapor decomposition of a low molecular weight silane compound onto the surface of a porous ceramic material in the presence of a platinum catalyst has been reported. However, in order to form a film according to this process, a metal must be present as a catalyst and accordingly the metal is present in a film. Therefore, when such a film is used for an electronically functional material, it might have a bad effect on the electric properties such that the relative permittivity is increased or the like. So, it is not preferable.
On the other hand, in JP1993-202478A, U.S Publication No. 2002098714, WO02/043119, and U.S. Pat. No. 6,348,725, a process for forming a film of a cyclic siloxane compound on a substrate by the plasma CVD technique has been reported. This process is to form a film by decomposing the cyclic siloxane compound into plasma and depositing it on a substrate. For this reason, the thus-obtained film has a very low porosity. For example, low relative permittivity that is required when the film is used for an interlayer insulating film in a semiconductor cannot be expected. Further, to generate plasma, a very expensive device is needed. Thus, this process is not economically desirable.
In order to enhance the film strength, there has been reported a process by excimer beam (EB) cure as described in Proceedings of IITC, p. 106 (2003). This process comprises placing a porous film on a single wafer heating stage installed in a vacuum chamber and performing EB cure at 350° C. in an argon atmosphere, under a condition of 10 Torr, thereby improving the mechanical strength by 1.5 times. However, this treatment has disadvantages such that expensive equipment is required and as a result of the treatment, a film thickness becomes reduced. A change in the film thickness is not preferable when such a film is used as an optically functional material or an electronically functional material.
As described above, the technology to prepare a porous film which satisfies both the hydrophobic property and the film strength has not yet been sufficient.